arXiv subjects

Masakiyo Kitazawa

Publications and source records attributed to Masakiyo Kitazawa.

At least 109 records · Page 6Linked to original sources

arXiv2014

Higher order fluctuations of conserved charges in heavy ion collisions

We investigate the effect of the global charge conservation on the cumulants of conserved charges in relativistic heavy ion collisions in a finite rapidity window by studying the time evolution of cumulants in the hadronic medium. It is argued that the global charge conservation does not affect the experimental result of the net-electric charge fluctuation observed by ALICE, because of the finite diffusion distance of charged particles in the hadronic stage.

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arXiv2014

Effects of global charge conservation on time evolution of cumulants of conserved charges in relativistic heavy ion collisions

We investigate the effect of the global charge conservation on the cumulants of conserved charges observed in relativistic heavy ion collisions in a finite rapidity window, $Δη$, with a special emphasis on the time evolution of fluctuations in the hadronic medium. It is argued that the experimental result of the net-electric charge fluctuation observed by ALICE does not receive effects from the global charge conservation, because of the finite diffusion distance of charged particles in the hadronic stage. We emphasize that the magnitude of the effect of the global charge conservation can be estimated experimentally by combining the information on the $Δη$ dependences of various cumulants of conserved charges, similarly to other dynamical properties of the hot medium.

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arXiv2014

Possible generation of anomalously soft quark excitations at nonzero temperature: Nonhyperbolic dispersion of parapion and van Hove singularity

We study the quark spectrum at finite temperature near and above the pseudocritical temperature of the chiral phase transition incorporating the effects of the collective modes with the quantum number of the sigma (parasigma) and pion (parapion) in a chiral effective model with a nonzero current quark mass. Below the pion zero-binding temperature where the pionic modes are bound, the quark self-energy has van Hove singularity induced by the scattering of quarks with the composite bound pions with a nonhyperbolic dispersion curve. This singularity is found to cause a drastic change in the quark spectrum from that in the mean field picture near the pseudocritical temperature: The quark spectrum has an unexpected sharp peak at an energy considerably lower than the constituent quark mass, while the spectrum approaches the mean field one at high temperatures. We clarify that the emergence of this anomalous structure of the quark spectral function originates from the composite nature of the pionic modes with a non-Lorentz invariant dispersion relation in the medium at finite temperature.

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arXiv2013

Nonequilibrium time evolution of higher order cumulants of conserved charges and event-by-event analysis

We investigate the time evolution of higher order cumulants of conserved charges in a volume with the diffusion master equation. Applying the result to the diffusion of non-Gaussian fluctuations in the hadronic stage of relativistic heavy ion collisions, we show that the fourth-order cumulant of net-electric charge at LHC energy is suppressed compared with the recently observed second-order cumulant at ALICE, if the higher order cumulants at hadronization are suppressed compared with their values in the hadron phase in equilibrium. The significance of the experimental information on the rapidity window dependence of various cumulants in investigating the history of the dynamical evolution of the hot medium created in relativistic heavy ion collisions is emphasized.

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arXiv2013

Shear viscosity to relaxation time ratio in SU(3) lattice gauge theory

We evaluate the ratio of the shear viscosity to the relaxation time of the shear flux above but near the critical temperature $T_c$ in SU(3) gauge theory on the lattice. The ratio is related to Kubo's canonical correlation of the energy-momentum tensor in Euclidean space with the relaxation time approximation and an appropriate regularization. Using this relation, the ratio is evaluated by direct measurements of the Euclidean observables on the lattice. We obtained the ratio with reasonable statistics for the range of temperature $1.3T_c \lesssim T \lesssim 4T_c$. We also found that the characteristic speed of the transverse plane wave in gluon media is almost constant, $v \simeq 0.5$, for $T \gtrsim 1.5T_c$, which is compatible with the causality in the second order dissipative hydrodynamics.

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arXiv2013

On the effect of secondary protons on baryon and proton number cumulants in event-by-event analysis

We investigate the effects of secondary (knockout) protons, which constitute about 20% of the observed protons at STAR, on the higher order cumulants of proton and baryon numbers measured by event-by-event analyses in relativistic heavy ion collisions. We argue that the contribution of this background effect on the cumulants is expressed by a simple formula, and that hence their effects can be removed in the experimental analysis. It is discussed that this background effect has non-negligible contribution to recently observed proton number cumulants at STAR, especially the third-order one, and that the removal of this effect is crucial to investigate the thermodynamical properties of the primordial hot medium appropriately.

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arXiv2012

Thermal mass and dispersion relations of quarks in the deconfined phase of quenched QCD

Temporal quark correlation functions are analyzed in quenched lattice QCD for two values of temperature above the critical temperature (Tc) for deconfinement, T=1.5Tc and 3Tc. A two-pole ansatz for the quark spectral function is used to determine the bare quark mass and the momentum dependence of excitation spectra on large lattices of size up to 128^3x16. The dependence of the quark correlator on these parameters as well as the finite volume dependence of the excitation energies are analyzed in detail in order to examine the reliability of our analysis. Our results suggest the existence of quasi-particle peaks in the quark spectrum. We furthermore find evidence that the dispersion relation of the plasmino mode has a minimum at non-zero momentum even in the non-perturbative region near Tc. We also elaborate on the enhancement of the quark correlator near the chiral limit which is observed at T=1.5Tc on about half of the gauge configurations. We attribute this to the presence of near zero-modes of the fermion matrix that are associated with non-trivial topology of the gauge configurations.

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arXiv2012

Relation between baryon number fluctuations and experimentally observed proton number fluctuations in relativistic heavy ion collisions

We explore the relation between proton and nucleon number fluctuations in the final state in relativistic heavy ion collisions. It is shown that the correlations between the isospins of nucleons in the final state are almost negligible over a wide range of collision energy. This leads to a factorization of the distribution function of the proton, neutron, and their antiparticles in the final state with binomial distribution functions. Using the factorization, we derive formulas to determine nucleon number cumulants, which are not direct experimental observables, from proton number fluctuations, which are experimentally observable in event-by-event analyses. With a simple treatment for strange baryons, the nucleon number cumulants are further promoted to the baryon number ones. Experimental determination of the baryon number cumulants makes it possible to compare various theoretical studies on them directly with experiments. Effects of nonzero isospin density on this formula are addressed quantitatively. It is shown that the effects are well suppressed over a wide energy range.

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arXiv2012

Revealing baryon number fluctuations from proton number fluctuations in relativistic heavy ion collisions

Baryon number cumulants are invaluable tools to diagnose the primordial stage of heavy ion collisions if they can be measured. In experiments, however, proton number cumulants have been measured as substitutes. In fact, proton number fluctuations are further modified in the hadron phase and different from those of baryon number. We show that the isospin distribution of nucleons at kinetic freezeout is binomial and factorized. This leads to formulas that express the baryon number cumulants sorely in terms of proron number fluctuations, which are experimentally observable.

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arXiv2010

Spectral properties of quarks above Tc -- thermal mass, dispersion relation, and self-energy --

Spectral properties of quarks above the critical temperature for deconfinement are analyzed in quenched lattice QCD on lattices of size 128^3x16. We study quark spectral function in energy and momentum space, focusing on the values of the thermal mass and the dispersion relations of normal and plasmino modes at nonzero momentum, as well as their spatial volume dependence. Our numerical result suggests that the dispersion relation of the plasmino mode has a minimum at nonzero momentum even near the critical temperature. The quark self-energy is also analyzed by using the analyticy of the inverse propagator, which is found to be consistent with the spectral function estimated by the two-pole ansatz.

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arXiv2009

Third moments of conserved charges as probes of QCD phase structure

The third moments of conserved charges, the baryon and electric charge numbers, and energy, as well as their mixed moments, carry more information on the state around the QCD phase boundary than previously proposed fluctuation observables and higher order moments. In particular, their signs give plenty of information on the location of the state created in relativistic heavy ion collisions in the temperature and baryon chemical potential plane. We demonstrate this with an effective model.

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arXiv2009

Evaluation of Israel-Stewart parameters in lattice gauge theory

Navier-Stokes equations are known as hydrodynamic equations which take account of effects of dissipations. There are, however, problems in the relativistic Navier-Stokes equations, i.e. the equations violate causality. Israel-Stewart equations, which evade the problems of Navier-Stokes equations by introducing new parameters, such as the relaxation times, have recently been used in describing the space-time evolution of the quark-gluon plasma produced in high energy heavy ion collisions. The viscosities and the relaxation times are related to each other by imposing entropy constraints on the system. According to Boltzmann-Einstein principle, the probability distribution of the fluctuation in the energy-momentum tensor is related to the entropy of the system. Applying this principle to the entropy in Israel-Stewart theory, one can obtain the ratios of the viscosities to the relaxation times. We evaluate the ratios of the viscosities to the relaxation times in SU(3) lattice gauge theory.

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arXiv2009

Third moments of conserved charges in QCD phase diagram

We point out that the third moments of conserved charges, the baryon and electric charge numbers, and energy, as well as their mixed moments, change their signs around the QCD phase boundary in the temperature and baryon chemical potential plane. These signs can be measured in relativistic heavy ion collisions, and will give clear information on the phase structure of QCD and the state of the system in the early stage of relativistic heavy ion collisions. The behaviors of these moments on the temperature axis and at small quark chemical potential can be analyzed in lattice QCD simulations. We emphasize that the third moments obtained on the lattice, together with the experimental results, will provide a deep understanding about the QCD phase diagram and the location of the state created in heavy ion collisions.

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arXiv2009

Spectral Properties of Quarks at Finite Temperature in Lattice QCD

We analyze the quark spectral function above and below the critical temperature for deconfinement and at finite momentum in quenched lattice QCD. It is found that the temporal quark correlation function in the deconfined phase near the critical temperature is well reproduced by a two-pole ansatz for the spectral function. The bare quark mass and momentum dependences of the spectral function are analyzed with this ansatz. In the chiral limit we find that even near the critical temperature the quark spectral function has two collective modes corresponding to the normal and plasmino excitations in the high temperature (T) limit. The pole mass of these modes at zero momentum, which should be identified to be the thermal mass of the quark, is approximately proportional to T in a rather wide range of T in the deconfined phase.

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arXiv2009

Quark propagator at finite temperature and finite momentum in quenched lattice QCD

We present an analysis of the quark spectral function above and below the critical temperature for deconfinement performed at zero and non-zero momentum in quenched lattice QCD using clover improved Wilson fermions in Landau gauge. It is found that the temporal quark correlation function in the deconfined phase near the critical temperature is well reproduced by a two-pole ansatz for the spectral function. This indicates that excitation modes of the quark field have small decay rates. The bare quark mass and momentum dependence of the spectral function is analyzed with this ansatz. In the chiral limit we find that the quark spectral function has two collective modes corresponding to the normal and plasmino excitations in the high temperature limit. Over a rather wide temperature range in the deconfined phase the pole mass of these modes at zero momentum, which corresponds to the thermal mass of the quark, is approximately proportional to temperature. With increasing bare quark masses the plasmino mode gradually disappears and the spectral function is dominated by a single pole. We also discuss quasi-particle properties of heavy quarks in the deconfined phase. In the confined phase, it is found that the pole ansatz for the spectral function fails completely.

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arXiv2008

Spectral properties of massless and massive quarks coupled with massive boson at finite temperature

We study the properties of massless and massive quarks coupled with a scalar and pseudoscalar boson at finite temperature in Yukawa models at the one-loop order. The behavior of the spectral function and the pole structure of the propagator are analyzed as functions of temperature $T$ and the quark mass $m_f$. It is shown that the three-peak structure of the spectral function found in a previous work for massless quarks is formed at temperatures comparable to the boson mass even for finite $m_f$, but gradually ceases to exist as $m_f$ becomes larger. We identify the three poles of the quark propagator corresponding to the collective excitations of the quark in the complex energy plane. It is shown that the three trajectories made by the poles along with a variation of $T$ undergo a structural rearrangement at a critical quark mass when $m_f$ is increased. This suggests that the physics content of the collective quark excitations is changed in a drastic way at this point. The results are nicely accounted for with the notion of the level mixing induced by a resonant scattering of the massive boson with quarks and holes of thermally excited anti-quarks.

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arXiv2007

How do diquark fluctuations and chiral soft modes affect di-lepton production in the deconfined phase?

We examine diquark fluctuations and chiral soft modes existing in the precritical region of color superconductivity and chiral transition, respectively, at finite temperature and density. We evaluate how they contribute to anomalous di-lepton production; although there appear peaks in the spectral function owing to the existence of the soft modes, the enhancement of the production rate may not be so prominent to be a clear signal of the phsase transitions.

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arXiv2007

Spectral properties of quarks above T_c in quenched lattice QCD

We analyze the quark spectral function above the critical temperature for deconfinement in quenched lattice QCD using clover improved Wilson fermions in Landau gauge. We show that the temporal quark correlator is well reproduced by a two-pole approximation for the spectral function and analyze the bare quark mass dependence of both poles as well as their residues. In the chiral limit we find that the quark spectral function has two collective modes which correspond to the normal and plasmino excitations. At large values of the bare quark mass the spectral function is dominated by a single pole.

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